The present invention relates to thermal detectors. More specifically, the present invention relates to tunable thermal detectors.
In recent decades, there has extensive work in the area of micromechanical filters and detectors. Most of these have been developed for the telecommunications industry. Common examples are vertical-cavity detectors, tunable vertical-cavity detectors, resonant-cavity enhanced photodetectors, and tunable resonant-cavity enhanced photodetectors. Less common are devices that use these concepts for longer wavelength applications. For example, an adaptation of telecommunications technology using a filter and semiconductor detector for the SWIR, MWIR, and LWIR has been given by Faraone, et. al., U.S. Patent Application Publication No. 2005/0226281, published Oct. 13, 2005 and entitled TUNABLE CAVITY RESONATOR AND METHOD OF FABRICATING SAME.
For thermal detection, filters must ultimately couple light into an absorption layer located somewhere in the system so that heat may be generated and detected by some means, such as by a microbolometers, golay cell, pyroelectric detector, etc. In these longer wavelength regions, there are significant applications for filters and detectors that can see only two or three broad bands. These devices are often called “two-color” or three-color” and they are often used for target identification and temperature measurement. Neikirk (see, A. S. Weling, P. F. Henning, D. P. Neikirk, and S. Han, “Antenna-coupled microbolometers for multispectral infrared imaging” Proceedings of the SPIE, vol. 6206, pp. 62061F-1 to 62061F-8, 2006; and S. Han, J.-Y. Jung, and D. P. Neikirk, “Multilayer fabry-perot Microbolometers for infrared wavelength selective detectors,” Proceedings of the SPIE, vol. 6206, pp. 62061G-1 to 62061G-7, 2006) and Butler (see, V. N. Leonov and D. P. Butler, “Two-color thermal detector with thermal chopping for infrared focal plane arrays,” Applied Optics, vol. 40, no. 16, 2001; and M. Almasri, B. Xu, and J. Castracane, “Amorphous silicon two-color microbolometer for uncooled IR detection,” IEEE Sensors Journal, Vol. 6, no. 2, pp. 293-300, 2006) have both proposed absorptive filters and electrostatically actuated thermal detectors that can perform such tasks. For narrower bands such as those used for performing denser multispectral target identification and detecting spectral signatures, as shown in Cole, et. al. U.S. Pat. No. 5,286,976, issued Feb. 15, 1994 and entitled MICROSTRUCTURE DESIGN FOR HIGH IR SENSITIVITY; U.S. Pat. No. 5,550,373, issued Aug. 27, 1996, and entitled FABRY-PEROT MICRO FILTER-DETECTOR; U.S. Pat. No. 6,816,636, issued Nov. 9, 2004, and entitled TUNABLE OPTICAL FILTER; U.S. Pat. No. 7,015,457, issued Mar. 21, 2006, and entitled SPECTRALLY TUNABLE DETECTOR; and U.S. Pat. No. 7,196,790, issued Mar. 27, 2007 and entitled MULTIPLE WAVELENGTH SPECTROMETER have proposed a tunable transmissive filter placed external to an absorbing thermal detector. The Cole et al. patents describe both electrostatic and piezoelectric actuation methods for such a device. Koskinen, U.S. Pat. No. 5,589,689, issued Dec. 31, 1996 and entitled INFRARED DETECTOR WITH FABRY-PEROT INTERFEROMETER has developed similar devices except that his absorption layer is on one of the mirrors and may boost the reflectivity of that mirror. Tai, et. al. U.S. Publication No. 2005/0017177, published Jan. 27, 2005 and entitled APPARATUS AND METHOD FOR SENSING ELECTROMAGNETIC RADIATION USING A TUNABLE DEVICE have modified this basic structure to use an absorption filter, as Neikirk has proposed for broadband filters, with a separate detector integrated on top.